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The stem as plant organ




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The attachment site of a leaf or bud on a stem is called a node, and the po rtion between nodes is called an internode. Most stems point upwards from the ground and are easily distinguished from other plant organs. Some stems, however, have an unusual shape or location which makes them more difficult to identify. Potato tubers, for example, appear root-like, but they are actually swollen underground stems specialised for food storage. All stems, of whatever size, shape, or location, are distinguishable as such by the presence of nodes and internodes.

The tissues and cells of a stem

In a dicotyledonous, non-woody (herbaceous) stem the epidermis is like that of a leaf: a single layer of cells perforated by stomata. The epidermis helps maintain the shape of the stem. It is covered with a waxy cuticle to reduce water loss. In woody stems of trees and bushes, the epidermis is replaced bybark consisting of many layers of dead cells. Bark is penetrated by small pores calledlenticels, through which gaseous exchange takes place. The lenticels usually appear as raised spots surrounded by a powdery and impermeable material.

Just inside the epidermis, a layer of collenchyma gives both support and flexibikity to the stem. Some collenchyma cells contain chloroplasts which make the stem appear green.

The inner parts of the stems of most non-woody plants consist of vascular bundles embedded in undifferentiated parenchyma cells. When fully inflated with water (turgid), the parenchyma cells press against the epidermis and collenchyma, strengthening the stem. The stems of trees and bushes are supported not by parenchyma but by rigid woody tissue which makes up the bulk of these stems. The woody tissue consists of xylem and associated cell such as fibres formed by a process called secondary growth. New wood is added outside the old wood each growing season to form annual growth rings, visible in transverse sections of the stems of trees and shrubs.

Vascular tissue in the stem takes the form of bundles containing phloem and xylem and reinforced with strong fibres. The xylem is located towards the inside of the stem and the phloem towards the outside. The tough rigid vascular bundles embedded in softer turgid parenchyma tissue have been likened to reinforced concrete, in which rigid steel girders are imbedded in softer concrete. This arrangement gives the stem strength and flexibility, making it well suited to resisting sideways bending in strong winds. The vascular bundles of dicotyledonous plants are arranged in a ring pattern around the outside of the stem, while in monocotyledons such as cacti the vascular bundles are scattered throughout the stem.



The stem centre is called the pith. It may consist of parenchyma cells for storage, or it may be devoid of cells, in which case it is called a pith cavity.

■ Glossary of essential terms for you to know:

N English term Russian equivalent
foremost самый главный
allow предоставлять, позволять
pollination опыление
dispersal рассеяние, разброс
replace заменить, замещать
attachment прикрепление
node узел, нарост, утолщение
internode междоузлие
point быть направленным, направляться
swollen разбухать, пухнуть
herbaceous травянистый
dicotyledonous двудольный
monocotyledonous однодольный
bark кора
stomata устьица
penetrate пронизываться, проникать
lenticel чечевичка
impermeable непронизаемый
embed вставлять, внедрять
inflate наполнять, надувать
turgid тургесцентный, набухший
rigid жесткий
transverse поперечный
tough несгибаемый, жесткий
liken уподоблять
girder брус, перекладина
scatter разбрасывать, рассеивать
phloem флоэма, луб
parenchyma паренхима  
resist сопротивляться  
pitch сердцевина  
devoid лишенный (чего-л.), свободный (от чего-л.)  
reinforce усиливать, подкреплять  
slit продольный разрез, щель  
           

■ Your Essential Assignments

I. Quick check

1. What distinguishes stems from other plant?

2. List the four functions structures?

3. How do parenchyma cells support herbaceous stems?

 

II. Fill in the missing words:

Term (verb) Noun Adjective
disperse ....... …….
attach ....... …….
…..... location …….
identify ....... …….
store ....... …….
maintain ....... …….
penetrate ….. …….
......... strength …….
grow ........ …….
add ....... …….

 

III. Use monolingual English dictionary and write down what could the words given below mean:

ground, growth, woody stem, non-woody stem, exchange, flexibility, growing season, resist, bundle.

 

IV. Find English equivalents to the following word combinations:

N Russian term English equivalent
1. 1 наземные растения  
2. получать оптимальную экспозицию к свету  
3. удерживать цветы и плоды  
4. эффективное опыление  
5. рассеивание семян  
6. заменять умершие клетки  
7. хранить (запасать) воду, питательные вещества  
8. трудно опознать (идентифицировать)  
9. эпидермис заменяется корой  
10. внутренние части стебля  
11. принимать форму пучков (узлов)  
12. придавать стеблю прочность и гибкость  

V. Give Russian equivalents to the following English terms:

N English term Russian equivalent
foremost organs of support  
attachment site of a leaf  
most stems point upward  
easily distinguished from  
unusual shape of location  
swollen underground stems  
maintain the shape of the stem  
reduce water loss  
rigid woody tissue  
in softer turgid tissues  

 

VI. Find synonyms among the pool of words:

Pool of words Synonyms
1)1.keep/2.replace/3.gain/4.store/5.substitude/6.obtain/  
2)1.ground / 2. appear / 3. soil / 4. emerge  
3)1.reinforce/2.like/3.situation/4.similar/5.position/6.strengthen  
4)1.support / 2. seem / 3. maintain/ 4. appear  
5)1.tough/2. strength/3.rigid/4.force/5.locate/6.situate  

 

VII. Answer the following questions. Use all information given before:

1. How is the stem centre called?

2. What kind of form does the vascular tissue take?

3. Where is the tough rigid vascular bundles embedded in?

4. How are vascular bundles arranged in the:

a) dicotyledonous plants

b) monocotyledonous plants?

5. Are the stems of trees supported by parenchyma?

6. What does the epidermis help?

7. What does the inner part of the stems consist of?

8. What is epidermis covered with?

9. Where do most stems point?

VIII. Match the sentence halves. Make complete sentences:

1. The stems of most plants are foremost organs of support because A. upwards from the ground.  
2. Most stems point B. parenchyma cells for storage or it may be devoid of cells.
3. Some stems have an unusual shape of location which C. vascular bundles embedded in undifferentiated cells.
4. All stems of whatever size, shape or location are distinguishable as such by D. gaseous exchange takes place.
5. The epidermis is covered with E. they lift terrestrial plants above the ground rising their leaves towards the sun.
6. Bark is penetrated by small pores called lenticels through which F. waxy cuticle to reduce water loss.
7. The inner parts of the stems of most non-woody plants consist of G. makes them more difficult to identify.
8. Vascular tissue in the stem take the form of H. the presence of nods and internodes.
9. The stem centre is called pitch. It may consist of I. bundles containing phloem and xylem and reinforces with strong fibres.

 




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